The maximum size of FAT32 is 2TB, SDXC cards can reach up to 2TB, so that's not an issue
Many microSDHC readers can read microSDXC cards if they are formated in FAT32 instead of ex-FAT.
But as far as I know nobody tried on the R0 yet.

Rockbox on the R0 works more or less like on the Fuze, you will hardly notice that is an app port.
The only major difference is that the port is not complete yet (that doesn't mean the support is worse, there is active support). FM radio is not yet implemented but should be soon and keymapping is not optimal for all plugins yet. Just a matter of time.

I don't have the R0 with me now but I don't see any reason why those plugins would be missing.

Storage things are managed by the linux kernel inside. It's not rockbox's duty. So indeed our kernel supports both FAT and FAT32 (no NTFS without an external module). SD(HC) are supported too. I cannot guarantee all the various models are finely supported, but it should be so and in any case the driver should be complete. The thing that is still to be fixed is that I'm not sure at the moment FAT16 will be correctly mounted since I'll do that in a script without too much control. Maybe this part should be fixed.
If anyone has suggestions, I used "-t vfat" as mount parameter. I don't know if that actually includes FAT16 or so...

Yes all plugins are supported. Just a small amount of them have little issues. The only completely non-working is Doom as far I know. But it could be debugged and fixed for sure